Additive manufacturing is a process by which an article is produced by joining materials together. Additive layer manufacturing (ALM) involves producing the article layer by layer using 3D model data. The process may be used for producing prototypes, samples, models, tooling or full scale components.
A known ALM technique uses a power beam, such as a laser or an electron beam for example, to selectively sinter powdered material. A layer of the powdered material is laid over a substrate (also known as a build plate, start plate, start platform or build platform) in a sintering chamber to provide a “powder bed”. The powder is applied using a re-coating blade. The power beam is used to selectively sinter the powder in a desired pattern by scanning across the surface of the powder bed. The power beam is controlled to produce the desired pattern according to cross-sections generated from 3D model data (e.g. a CAD file) of the article.
After each cross-section is scanned and the melted powder has solidified, the substrate is lowered by one layer thickness (typically of the order of 0.1 mm) to prepare for growth of the next layer. Another layer of the powder is applied on top of the preceding layer in preparation for sintering. The process is repeated until the article is completed. Thus as the process proceeds, a sintered article is constructed, supported by unconsolidated powder. After the article has been completed it is removed from the substrate and the unconsolidated powder is typically recycled to produce another article by the same ALM process.
Selective sintering may be used to produce shaped articles from powders of some metals, ceramics, glasses and thermoplastic polymers.
Due to the thermal energy involved in sintering the powder it is generally necessary to constrain the article during the build against thermal residual stresses which tend to make the article curl up, clashing with the re-coating blade and causing the build to fail. The article is generally constrained by fusing it to the substrate. In conventional processing a thick, solid substrate is used and the first layer of powder is fused to this, with subsequent layers being built on top. The fusion to the substrate provides the desired constraint in the z-axis (through layers) but also applies an additional constraint in the x-y plane, resulting in high residual stresses in the article which cannot be relieved during the build process. This can lead to bending of the substrate, and distortion and cracking of the article.
Currently, there are some ALM processes that make use of either a heated sintering chamber, or a heated substrate in order to reduce the level of differential shrinkage between the article and the substrate. This can reduce the level of residual stress, but has other drawbacks. The heated nature of the process results in a significant increase in system complexity, requiring heat shielding of sensitive electronics. It also changes the environment of the process, changing cooling characteristics which can be advantageous for some alloys, but can have a severe detrimental effect on other, e.g. Scalmalloy™, which requires fast solidification and high cooling rates.